One well known method of making glass sheets involves transporting the glass sheets through a furnace on a horizontal roller conveyor to heat the glass sheets to a temperature suitable for further processing of the glass sheets, such as bending and/or tempering or annealing. Since the glass sheets are typically heated to the softening point of the glass during these processes, conveyance and processing of the glass sheet may cause undesirable changes in the glass sheets, resulting in distortion in the images emitted or reflected by the glass when it is mounted, for example, in a building.
The distortion of images is typically caused by variations in the flatness of the glass surface. Isolated convex or concave lenses formed on the glass surface may cause distortion of images viewed through the glass and may render the glass unacceptable for architectural use.
One type of distortion commonly found in glass sheets processed on a roller conveyor hearth is a cylindrical sinusoidal wave of corrugation (or roller wave) resulting from deformation of the softened glass sheet as it is being supported by the rollers. It is desirable to quantify this characteristic in the glass in order to establish a quality standard for grading the glass during production, for example, to determine whether the glass has optics suitable for architectural use.
It is well known to provide optical inspection devices for measuring different characteristics of glass sheets. U.S. Pat. No. 3,591,293, issued to Maltby, Jr., et al. for an "Apparatus for Determining the Thickness of a Transparent Material by Measuring the Time Interval Between Impingement of Front and Back Surface Reflections on the Detector," utilizes a beam of light projected toward a glass surface and reflected from each of the opposite surfaces of the glass sheet toward and into a chopper assembly to continually monitor the thickness and/or width and position of a glass ribbon moving through the annealing end of a glass-making machine.
U.S. Pat. No. 3,788,750, issued to Maltby, Jr., et al. for "Inspecting Glass" discloses a method and apparatus for determining imperfections in flat glass sheets comprising a light source directing a pair of incident beams of light against the sheet when the sheet is supported on an inspection table, a chopper assembly for intercepting the reflected light beams and sequentially chopping the beams at a constant rate, and light sensitive means in the path of the chopped beams and producing an electrical signal of two pulses separated in time by an amount proportional to the spacing between the beam, and thereafter graphically recording the readings on a strip chart as an indication of position and focal length of lenses on the surface of the glass sheets.
U.S. Pat. No. 4,585,343, issued to Schave et al. for an "Apparatus and Method for Inspecting Glass", discloses an inspection apparatus for detecting surface distortion including a light source mounted to direct a beam of light toward the surface of the glass sheet, and a light detector mounted to receive the reflected beam and generate an output signal representing the width of the light pattern, which width is a function of the surface distortion of the sheet. This patent also discloses the use of a microcomputer connected to a display device.
While existing inspection devices provide some useful data on the characteristics and properties of glass sheets, none of the prior art devices provide the advantages of the present invention as is hereinafter more fully described.